The suprachiasmatic nucleus (SCN) is the master circadian clock in mammals. Individual SCN neurons are cell-autonomous oscillators with diverse free-running periods of activity (19 to 28 hours). Intracellular communication is required to synchronize these individual neuronal oscillators to a period of 24 hrs for expression of physiological circadian rhythms. The signaling mechanisms mediated in part by GABA and vasoactive intestinal peptide (VIP) neurotransmission that synchronize neuronal oscillators remain obscure. In other brain regions, GABAA receptor-mediated neurotransmission is characterized by a great diversity in physiological and pharmacological properties. These diverse properties make critical contributions to coordinating interneuronal communication and neural network activity. The long-term goal of our research is to identify the signaling mechanisms mediating intercellular communication and oscillator synchronization in the SCN. Neurotransmission mediated by GABAA receptors in the SCN has been described in very simplistic terms - it is inhibitory or excitatory and is blocked by GABAA receptor antagonists. We will use electrophysiological and imaging techniques to examine the diversity of GABAergic neurotransmission in the SCN assess the contribution of these properties to the excitability and synchronization of SCN neurons. A second critical step in SCN neuronal synchronization involves VIP activation of VPAC2 receptors. We will use an innovative combination of live-cell reporters for cyclic AMP and protein kinase A together with electrophysiological recording techniques to identify the signaling mechanisms mediating the activity of VIP on SCN neurons. The Specific Aims of the proposal are: 1) Characterize the physiological and pharmacological properties of GABAA receptor-mediated fast synaptic and tonic neurotransmission and its role in the regulation of action potential firing of SCN neurons. 2) Determine the contribution of fast synaptic and tonic GABAA receptor-mediated neurotransmission to the synchronization of SCN neurons. 3) Identify the signaling mechanisms activated by VIP binding to VPAC2 receptors expressed in SCN neurons. 4) Examine the interaction between GABAergic neurotransmission and VIP signaling pathways in the SCN. The completion of these studies will lead to better understanding of how circadian rhythms are generated and maintained. Many neuroactive compounds including benzodiazepines, steroids, and ethanol alter the timing of the circadian clock and disrupt sleep and mood. These studies will provide new information on the mechanism of action of these compounds leading to the rationale development of better therapeutic agents.